The present invention relates to a method and apparatus for detecting irregularities in a coating on a substrate. More particularly, the present invention relates to the inspection of painted or lacquered surfaces, such as surfaces on container closures including metal screw and lug cap components. Such inspection is performed to detect physical damage and the presence of contamination.
Currently in the container closures production process, considerable effort is expended in inspecting each closure for the presence of surface coating defects and contaminants. The coatings generally include a transparent carrier and pigment particles which scatter incident light. Typical of such coatings are enamels, latex paints, epoxy coatings and lacquers, among others. These can generally be called solid solutions and mixtures.
Coating defects can include the presence of alien materials such as stains and residual sealing materials, physical damage from scratching or abrasion, and pin holes. Cosmetic and functional properties of the closure can be adversely affected by alien material or by flaws in the protective surface. Damage to the coating may expose the underlying closure material to corrosion which may lead the closure to fail.
To maintain high product quality, it is necessary to separate defective caps from good caps. Because some of these defects, such as pin holes, are extremely small, naked eye inspection is not only inefficient, but is also relatively ineffective. Closures also generally contain some embossed structure which must be ignored by an inspector. Optical techniques that rely on the contrast difference between defective and good cap areas can misread these embossed structures, since they also produce contrast differences.
Thus, there is a need for a simple, inexpensive, reliable method and apparatus for inspecting coatings on a substrate to detect irregularities.
The use of polarized light to aid in inspections of surfaces is not, per se, new. For instance, U.S. Pat. No. 2,947,212 to R. C. Woods discloses a method for inspecting striated sheet metal moving parallel to the striations in which the metal surface is exposed to polarized light. The striations, consisting of microscopic peaks and valleys in the metal surface, cause non-specular reflection into a detector. The reflected light is filtered to pass only the polarized light. Variations in the intensity of the reflected beam represent variations in the striation.
In U.S. Pat. No. 3,904,293 to Gee, polarized light is beamed at a road surface. The reflected light has a certain degree of depolarization indicative of the surface texture of the road.
In U.S. Pat. No. 4,015,127 to Sharkins, a beam of non-polarized infrared radiation is directed at a coating on a metal substrate, at Brewster's Angle, so that the beam is plane-polarized if reflected from the top surface of the coating. Radiation not so reflected from the top surface of the coating penetrates and is partially absorbed by the coating and partially reflected at the interface of the coating and substrate. The polarized light is filtered out of the reflected beam so that the intensity of the portion of the incident beam penetrating the coating can be monitored without including the reflections from the top surface. The thickness of the coating can be determined by correlating it with the attenuation in beam intensity caused by absorption in the coating. Sharkins also discloses other combinations of polarizing filters and applications of Brewster's Angle to eliminate the unwanted polarized reflected beam.
None of these prior patents disclose the simple method or apparatus of the present invention for ascertaining the presence of irregularities in a coating. In particular, none of these patents discloses detection of surface contaminants while disregarding desirable embossing.